This application incorporates by reference of Taiwan application Serial No. 090100392, filed on Jan. 8, 2001.
1. Field of the Invention
The invention relates in general to a method of driving an active matrix electro-luminescent display, and more particularly to a method of driving an active matrix electro-luminescent display for preventing threshold voltage shift of thin film transistors in the active matrix electro-luminescent display.
2. Description of the Related Art
Active matrix electro-luminescent (AMEL) displays are generally used for small size displays, e.g., 1.3xe2x80x3xc3x971.2xe2x80x3, with high resolution. The AMEL displays employ organic light emitting diodes (O-LEDs) to generate optical signals. The brightness of an O-LED depends on the current flowing through itself. In addition, various types of transistors can be used as the active components to drive the O-LEDs. Among them, poly-Si thin film transistors (poly-Si TFT) are widely used. On the other hand, in thin film transistor liquid crystal displays (TFT-LCDs), amorphous Si thin film transistors (a-TFT) are widely used because of fewer masks for manufacturing, low film formation temperature, and low manufacturing cost. However, either poly-Si TFT or a-TFT has the problem that the conducting current decreases due to the threshold voltage shift after a long working time. This problem becomes serious especially while a-TFTs are used. Thus, AMEL displays rarely employ a-TFT.
Referring to FIG. 1, it shows a pixel array of O-LEDs for an AMEL display. The AMEL display has M scan lines and N data lines, forming a display of Mxc3x97N pixels. A video sequence having a number of consecutive frames can be displayed in the AMEL display with the Mxc3x97N pixels. Each pixel, denoted as P, has an O-LED, denoted as D, driven by thin film transistors Ta, Tb, and a capacitor C, wherein the source or drain of the transistor Ta is coupled to one of the data lines and the gate of the transistor Ta is coupled to one of the scan lines.
For example, in a pixel in FIG. 1, such as pixel P(1, 1), or P11, the gate of a transistor Ta(1, 1), or T11a, is connected to a scan line, Scan(1), or S1, and the source (or drain) of the transistor Ta(1, 1) is connected to a data line, Data(1), or D1, and the drain (or source) of the transistor Ta(1, 1) is connected to capacitor C(1, 1), or C11, and the gate of a transistor Tb(1, 1), or T11b. The drain of the transistor Tb(1, 1) is connected to an O-LED D(1, 1), or D11, while the source of the transistor Tb(1, 1) is connected to a direct current (DC) voltage source VDD, wherein the transistor Tb(1, 1) is an N-type transistor.
Referring to FIG. 2, it illustrates waveforms for driving the circuit shown in FIG. 1. The time for the AMEL display to display a frame is defined as a frame time interval I. A conventional method for driving an AMEL display is as follows. Firstly, scan each of the scan lines sequentially. That is, apply a pulse with a positive voltage to the scan lines, Scan(1) to Scan(M), sequentially so as to turn on the transistors Ta of all of the pixels on each scan line. Simultaneously, as the transistors Ta are turned on, data signals representative of different required brightness are applied to the data lines associated with the pixels to emit light. In addition, different signal levels of the data signals correspond to the brightness for the pixels.
For example, at time t1, while a pulse 202 is applied to the scan line Scan(1) so as to turn on transistors Ta(1, 1), Ta(1, 2), and Ta(1, 3), data signals with signal levels V(1, 1), V(1, 2), and V(1, 3) are applied to data lines Data(1), Data(2), and Data(3), as shown in FIG. 2. As the pulse 202 is applied to the scan line Scan(1), capacitors C(1, 1), C(1, 2), and C(1, 3) are being charged so that voltages of nodes N(1, 1), N(1, 2), and N(1, 3) approach the signal levels V(1, 1), V(1, 2), and V(1, 3) and transistors Tb(1, 1), Tb(1, 2), Tb(1, 3) are turned on. At the same time, current flows from the DC current source VDD through the transistors Tb(1, 1), Tb(1, 2), Tb(1, 3), O-LEDs D(1, 1), D(1, 2), and D(1, 3) so that the O-LEDs D(1, 1), D(1, 2), and D(1, 3) of the pixels P(1, 1), P(1, 2), and P(1, 3) emit light with different brightness. Since the signal levels V(1, 1), V(1, 2), and V(1, 3) are different, the current flowing through the O-LEDs D(1, 1), D(1, 2), and D(1, 3) are different. As a result, the brightness for the pixels P(1, 1), P(1, 2), and P(1, 3) are different.
At time t2, although the voltage applied to the scan line Scan(1) is changed to a low level and the transistors Ta(1, 1), Ta(1, 2), and Ta(1, 3) are turned off, the capacitor C(1, 1), C(1, 2), and C(1, 3) store charges and nodes N(1, 1), N(1, 2), and N(1, 3) maintain in a high level, the transistors Tb(1, 1), Tb(1, 2), Tb(1, 3) are still in a turn-on state and the O-LEDs D(1, 1), D(1, 2), and D(1, 3) continue to emit light. Thus, at time t2, the pixels P(1, 1), P(1, 2), and P(1, 3) keeps in a state for displaying. After the frame time interval I for the current frame elapses, the state of the pixels will be changed.
During a frame time interval I, threshold voltage shift may occur in the transistors Tb and would degrade the display quality. To illustrate this phenomenon, a duty ratio for a transistor is defined as a ratio of the period during which a transistor is in a turn-on state during a frame time interval to the length of the frame time interval I. For example, during the frame time interval for one frame, the pixel P(1, 1) is selected for displaying. As described above, the voltage across the capacitor C(1, 1) keeps in the high level V(1, 1) during the frame time interval and the gate of the transistor Tb(1, 1) thus remains a high level and has a current flowing through it. At the same time, the O-LED D(1, 1) emits light because of current flow through it. In this situation, the duty ratio for the transistor Tb(1, 1) is one since the transistor Tb(1, 1) remains turned on during the entire frame time interval. Unfortunately, threshold voltage shift may occur in that case. Besides, as will be explained below, the effect of threshold voltage shift occurred in the transistor Tb(1, 1) may seriously degrade the display quality.
The cause of threshold voltage shift mentioned above is described as follows. If the transistor Tb(1, 1) is an amorphous Si thin film transistor, its gate terminal is covered with an isolation layer of SiN formed at a low temperature. When the gate terminal remains in the high level state, the gate terminal will attract ions within the isolation layer of SiN and that will result in an increased voltage for the transistor Tb(1, 1) to conduct. In other words, the threshold voltage for the transistor Tb(1, 1) increases. In that case, as the capacitor C(1, 1) applies a fixed voltage to the transistor Tb(1, 1) the current flowing through the transistor Tb(1, 1) decreases, thereby reducing the brightness for the O-LED D(1, 1). The threshold voltage shift occurs in the transistor Tb with its duty ratio of one. Furthermore, the amount of brightness reduction for each pixel P is different since the voltage across the capacitor C associated with the transistor Tb of the pixel P is different. Thus, the brightness for the AMEL display may vary inconsistently and accordingly degrade the display quality. The problem due to threshold voltage shift may also occur in poly-Si TFT and degrades the display quality especially after the display is used for a long time.
It is therefore an object of the invention to provide a method of driving an active matrix electro-luminescent (AMEL) display for preventing the effect of the threshold voltage shift of thin film transistors in order to stabilize the brightness of the AMEL display and enhance the display quality.
The invention achieves the above-identified objects by providing a method of driving an AMEL display. The AMEL display includes M scan lines, N data lines, and Mxc3x97N pixels, wherein the Mxc3x97N pixels are capable of displaying a video signal having a plurality of consecutive frames. A frame time interval is defined as the time required for displaying one of the frames. The frame time interval has at least a first sub-interval and a second sub-interval. In addition, the pixels includes a pixel (p, q), wherein p is a positive integer not greater than M and q is a positive integer not greater than N. The pixel (p, q) includes a first transistor, a second transistor, a capacitor, and an organic light emitting diode (O-LED). The first transistor has a source/drain terminal coupled to the q-th data line and a gate terminal coupled to the p-th scan line. The second transistor is coupled to the first transistor. When a first pulse is applied to the p-th scan line, the first transistor turns on and transmits a data signal on the q-th data line to the gate of the second transistor, wherein the data signal determines the operating of the second transistor. The capacitor is coupled to the gate terminal of the second transistor. The O-LED is coupled to the source/drain terminal of the second transistor, wherein the O-LED emits light when the second transistor operates with current flowing through its source and drain. Therefore, the brightness of the O-LED corresponds to a signal level of the data signal. The method includes the steps as follows. First, during the first sub-interval, apply a first pulse to the scan lines sequentially and apply corresponding data signals to the data lines. Next, during the second sub-interval, apply a second pulse to the scan lines sequentially so as to turn on the first transistors and apply a prevention signal to the data lines so as to turn off the corresponding second transistors.